Optical communication system, device and method employing advanced coding and high modulation order

CROSS REFERENCE TO RELATED APPLICATIONS

This application relates to a concurrently filed, co-pending, and commonly assigned U.S. patent application Ser. No. 12/959,874, entitled “System, Devices and Methods for Subcarrier Recovery at Local Oscillator Frequency in Optical OFDM System” by Qi Yang et al., the disclosure of which is hereby incorporated by reference herein in its entirety.

FIELD OF THE INVENTION

The present invention relates generally to optical communication systems, and more particularly to transmitters and receivers that operate at a high-speed transmission per single channel over a long-haul distance.

DESCRIPTION OF RELATED ART

Research on optical transmission has generally been concentrated on the following four aspects: increase a data rate, increase a transmission reach, increase spectrum efficiency, and improve receiver sensitivity. With fast growing of optical networks, necessity and importance for the above-mentioned four aspects becomes much obvious.

Research groups in recent time have shown over 1-Tb/s signal transmission via a 1000-km (one thousand kilometer) standard single mode fiber (SSMF) using the Wavelength-Division Multiplexing (WDM) technique. For instance, S. L. Jansen in 2008 showed 10×121.9-Gb/s transmission over a 1000-km SSMF. A. Sano showed 30×100-Gb/s all-optical OFDM transmission over a 1300-km SSMF. Although the WDM technique can effectively increase a line rate for optical communication, a fundamental enhancement is increasing the data rate per single channel. In the last decade, a signal with 100-Gb/s per single channel has been widely studied. To keep up with ever-increasing bandwidth demand from the Internet traffic, it is widely believed that 400-Gb/s or 1-Tb/s transport is poised to emerge in the next decade. Several experimental demonstrations have shown a line rate per single wavelength beyond 1-Tb/s. Roman Dischler et al. demonstrated a 1.2-Tb/s per single channel coherent optical OFDM transmission over a 400-km SSMF. And Yiran Ma et al. showed 1-Tb/s per single channel coherent optical OFDM transmission over a 600-km SSMF, which is the longest reach using a SSMF link so far. One of the main limitations is enhanced nonlinearity due to continuous ultra-broad spectrum of a Tb/s signal which results in reach reduction compared to current 100-Gb/s transport. Although in the same year, Xiang Liu and S. Chandrasekhar demonstrated a 1.2-Tb/s over 7200-km transmission, such work is largely supported by Raman amplification and an ultra-large-area fiber. However, existing fiber plants are mainly constructed by SSMF. And the main amplification joints in the existing fiber links are using Erbium doped fiber amplifiers (EDFAs). It is desirable for the emerging Tb/s transport to be compatible with the existing fiber plants. It is, therefore, of high importance to improve sensitivity of a receiving device so that an optical signal can be recovered in the presence of linear optical amplifier noises and fiber nonlinearity.

One key approach to improve the system sensitivity is using high-performance error correction codes (ECC). For instance, Reed-Solomon (RS) codes or BCH codes can be used to correct a noised signal with a bit error ratio (BER) at ˜1×10⁻³level. However, a common way to use such kinds of codes introduces some overhead. The increased overhead (lower rate) codes further improve the system performance, but also require higher bandwidth that adversely affect the spectrum efficiency and the system cost. It is thus preferable to improve the system performance using strong FEC codes without extending the signal bandwidth. Another coding scheme, named Trellis-coded modulation (TCM), is using overhead to map a signal onto high order modulation (i.e., perform high order modulation on a signal). By doing so, a distance of a signal constellation is extended. Qi Yang showed a 1-Tb/s TCM coded coherent optical OFDM transmission. That work extended the original transmission with quadrature phase shift keying (M-PSK, M=4) constellation onto 8-PSK (M-PSK, M=8). The optical signal-to-noise ratio (OSNR) achieves 2.6 dB improvements at back-to-back (0-km transmission). However, such kind of code has its limitation that it is very sensitive to phase issue on the constellation, which means the improvement may degrade during the long-haul transmission. Xiang Liu showed a 44-Gb/s trellis-coded 32-QAM CO-OFDM transmission over a 990-km SSMF, which has improvement of 1.2 dB. Compared to the work done by Qi Yang in 2010, which was published in OECC'2010, the same transmission at back-to-back can have 3.4 dB improvement. Although such scheme can be used to improve the system performance, it is not a good choice for long-haul transmission due to its ultra sensitivity to phase noises. To achieve good improvement, the computation complexity of TCM becomes very heavy. Thus, the preferred ECC codes should have a strong coding ability for various transmission cases, and practical computation complexity, which are essential for 1-Tb/s signal to reach an over 1000-km distance.

In a conventional coherent optical transmission with either single carrier scheme or OFDM scheme, QPSK is the most frequently used modulation format, which has several advantages over the others. The spectrum efficiency can achieve more than 3 bit/s/Hz, which is greater than BPSK, conventional binary NRZ format, etc. This modulation format has good receiver sensitivity, while having a low computation complexity. However, to realize more than 1-Tb/s transmission using such modulation format for over 1000-km SSMF reaches, the conventional technique for QPSK modulation appears to be insufficient.

Accordingly, it is desirable to have an optical communication system, device, and method that improve the system sensitivity for one or several Tb/s long-haul transmissions.

SUMMARY OF THE INVENTION

The present invention provides a transmitting device, a receiving device, an optical communication system, and methods, which improve the system sensitivity and achieve a high speed (e.g., over 1-Tb/s) transmission per single channel over a long-haul distance (e.g. over 1000-km).

In one embodiment of the invention, a transmitting device transmits an optical signal containing data, comprising an optical tone generator for generating at least one optical tone; one or more encoders, each encoder performing advanced coding on an associated data signal to generated an associated coded data signal, the associated coded data signal carrying a portion of the data; one or more mappers, each mapper coupled to an associated encoder for receiving the associated coded data signal, each mapper performing high order modulation on the associated coded data signal; and an up-converter, coupled to the one or more mappers, for up-converting the associated high-order-modulated data signal in each mapper into the optical signal to be outputted through the at least one optical tone.

In one embodiment of the invention, there is provided a receiving device for receiving an optical signal containing data subjected to advanced coding and high order modulation, comprising a down-converter for down-converting the optical signal into at least one data signal; at least one demapper for performing high order demodulation corresponding to the high order modulation on the at least one data signal; and at least one decoder for performing advanced decoding corresponding to the advanced coding on the at least one data high-order-demodulated signal to recover the data.

In one embodiment of the invention, there is provided an optical communication system comprising the above transmitting device and the above receiving device.

In one embodiment of the invention, a method is provided for transmitting an optical signal containing data, comprising generating at least one optical tone; performing advanced coding on at least one data signal respectively, each of the at least one data signal carrying a part of the data; performing high order modulation on the at least one coded data signal; and up-converting the at least one high-order-modulated data signal into the optical signal to be outputted through the at least optical tone.

In one embodiment of the invention, a method is provided for receiving an optical signal containing data subjected to advanced coding and high order modulation, comprising: down-converting the optical signal into at least one data signal; performing high order demodulation corresponding to the high order modulation on the at least one data signal; and performing advanced decoding corresponding to the advanced coding on the at least one high-order-demodulated data signal to recover the data.

The transmitting device, the receiving device, the optical communication system, and the methods according to the present invention apply advanced coding, such as the low density parity-check coding (LDPC) or Turbo coding, and high order modulation on data signals to be transmitted, and thereby may achieve high speed (e.g., over 1-Tb/s) transmission per single channel over a long-haul distance (e.g. over 1000-km) with error-free recovery.

The structures and methods of the present invention are disclosed in the detailed description below. This summary does not purport to define the invention. The invention is defined by the claims. These and other embodiments, features, aspects, and advantages of the invention will become better understood with regard to the following description, appended claims and accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is better understood upon consideration of the detailed description below and the accompanying drawings, in which:

<figref idref="DRAWINGS">FIG. 1</figref> is a system diagram illustrating the configuration of a high speed long-haul optical communication system in accordance with a first embodiment of the present invention.

<figref idref="DRAWINGS">FIG. 2A</figref> is a block diagram illustrating an optical tone generator as shown in <figref idref="DRAWINGS">FIG. 1</figref> in accordance with the present invention; <figref idref="DRAWINGS">FIG. 2B</figref> illustrates three optical tones generated by applying one optical tone generating unit in the optical tone generator in accordance with the present invention; and <figref idref="DRAWINGS">FIG. 2C</figref> illustrates multiple optical tones generated by applying two cascaded optical tone generating units in the optical tone generator in accordance with the present invention.

<figref idref="DRAWINGS">FIG. 3</figref> is a block diagram illustrating the up-converter as shown in <figref idref="DRAWINGS">FIG. 1</figref> in accordance with the present invention.

<figref idref="DRAWINGS">FIG. 4</figref> is a block diagram illustrating the down-converter shown in <figref idref="DRAWINGS">FIG. 1</figref> in accordance with the present invention.

<figref idref="DRAWINGS">FIG. 5</figref> is a more detailed block diagram illustrating a down-converting unit as shown in <figref idref="DRAWINGS">FIG. 4</figref> in accordance with the present invention.

<figref idref="DRAWINGS">FIG. 6</figref> shows a table presenting BER-versus-OSNR performance for 1.08-Tb/s coherent optical OFDM over a 1040-km SSMF transmission under OSNR of 28.7 dB.

<figref idref="DRAWINGS">FIG. 7</figref> is a flow chart illustrating a method of transmitting an optical signal according to a first embodiment of the invention.

<figref idref="DRAWINGS">FIG. 8</figref> shows a flow chart of a method of receiving an optical signal according to a first embodiment of the invention.

<figref idref="DRAWINGS">FIG. 9</figref> shows a schematic configuration of a high speed long-haul optical communication system according to a second embodiment of the present invention.

<figref idref="DRAWINGS">FIG. 10</figref> is a block diagram illustrating an optical signal generator as shown in <figref idref="DRAWINGS">FIG. 8</figref> in accordance with the present invention.

<figref idref="DRAWINGS">FIG. 11</figref> is a block diagram illustrating a sub-band signal generating unit as shown in <figref idref="DRAWINGS">FIG. 9</figref> in accordance with the present invention.

<figref idref="DRAWINGS">FIG. 12</figref> is a block diagram illustrating a signal processor shown in <figref idref="DRAWINGS">FIG. 8</figref> in accordance with the present invention.

<figref idref="DRAWINGS">FIG. 13</figref> is a flow chart illustrating a method of transmitting an optical signal according to a second embodiment of the invention,

<figref idref="DRAWINGS">FIG. 14</figref> is a flow chart illustrating a method of receiving an optical signal according to a second embodiment of the invention.

DETAILED DESCRIPTION

A description of structural embodiments and methods of the present invention is provided with reference to <figref idref="DRAWINGS">FIGS. 1-14</figref>. It is to be understood that there is no intention to limit the invention to the specifically disclosed embodiments but that the invention may be practiced using other features, elements, methods and embodiments. Like elements in various embodiments are commonly referred to with like reference numerals. Various exemplary embodiments of the present invention will now be described more fully with reference to the accompanying drawings in which only some exemplary embodiments are shown. Specific structural and functional details disclosed herein are merely representative for purposes of describing exemplary embodiments. The present invention, however, may be embodied in many alternate forms and should not be construed as limited to only the exemplary embodiments set forth herein.

An optical communication system, a transmitting device, and a receiving device according to a first embodiment of the invention are described in detail with reference to <figref idref="DRAWINGS">FIGS. 1-3</figref>.

Referring now to <figref idref="DRAWINGS">FIG. 1</figref>, there is shown a system configuration of a high speed (e.g. over 1-Tb/s) long-haul (e.g., over 1000-km) optical transmission system <b>10</b> according to a first embodiment of the present invention. The optical communication system <b>10</b> includes a transmitter (also referred to as a transmission device, or a transmitting device) <b>20</b>, an optical transmission channel <b>30</b>, and a receiver (also referred to as a receiving device) <b>40</b>. The transmitter <b>20</b> is coupled to the optical transmission channel <b>30</b>. The optical transmission channel <b>30</b> in turn is coupled to the receiver <b>40</b>.

The transmitter <b>20</b> transmits an optical signal containing data to the receiver <b>40</b> via the optical transmission channel <b>30</b>. The transmitter <b>20</b> includes an optical source <b>21</b>, an optical tone generator <b>22</b>, a data source <b>23</b>, one or more (N_t) encoders <b>24</b>, N_tmappers <b>25</b>, and an up-converter <b>26</b>, where N_t≧1. The optical source <b>21</b> can originate from various sources, such as a single laser source, which generates a single continuous-wave (CW) optical carrier at a predetermined wavelength. The optical tone generator <b>22</b> receives a single optical carrier generated by the optical source <b>21</b>. The optical tone generator <b>22</b> generates N_toptical tones from the single optical carrier and sends the N_toptical tones to the up-converter <b>26</b>. The up-converter <b>26</b>, which is described in detail with respect to <figref idref="DRAWINGS">FIG. 3</figref>, includes N_toptical modulators where each optical modulator receives a respective optical tone from the optical tone generator <b>22</b>. The generated tones refer to optical carriers at different wavelengths. An example of the optical tone generator is shown in <figref idref="DRAWINGS">FIG. 2</figref>. For example, a single laser source generates 50 tones, and each tone is used to carry a 21.6-Gb/s signal, which collectively yields a final net rate of 1.08-Tb/s.

The data source <b>23</b> is a source which provides the data (e.g., data bits) to be transmitted in a form of data signals. The date source <b>23</b> may be any type of data source commonly known in the art. The data source <b>23</b> provides N_tdata signals to the respective encoders <b>24</b>. Specifically, the data (bits) to be transmitted are divided into N_tparts, so that each part of the data is carried by one data signal, and the N_tdata signals carry all the data (bits) to be transmitted. In the 1.08-Tb/s example, the N_tis 50.

The N_tencoders <b>24</b> perform advanced coding on the N_tdata signals, respectively, and then provide the coded data signals to corresponding mappers. Preferably, the advanced coding used here may be coding with a net coding gain above 5 dB at BER=10⁻¹³compared to BER-versus-OSNR performance of un-coded transmission, and may have a low coding rate, for example, a net coding rate between 20%-85%. The encoders <b>24</b> may encode the data signals by using a strong error correction coding (ECC) scheme, such as low-density parity-check coding scheme. Another example of the advanced coding is Turbo coding scheme. It is to be noted that, the advanced coding scheme is not limited to the low-density parity-check coding scheme and the Turbo coding scheme, and other advanced error correction coding scheme currently known or developed in the future may also be employed as the advanced coding scheme. In the 1.08-Tb/s example, the used ECC scheme is LDPC coding scheme with a code rate of 50%, and the code size is (15120, 7560).

The N_tmappers <b>25</b> map the N_tcoded data signals onto high order modulation, i.e. perform high order modulation on the N_tcoded data signals, respectively. For example, the mappers <b>25</b> may employ M-PSK or M-QAM modulation, where M is a modulation order equal to or larger than 4 (M≧4) as the high order modulation. By doing so, even if the ECC coding rate at the encoders <b>24</b> is low, the spectrum efficiency of the optical communication system <b>10</b> maintains at least a conventional level (e.g., the spectrum efficiency may be greater than 2.5 bit/s/Hz). In the 1.08-Tb/s example, the constellation is using 16-QAM, where M is equal to 16. When a 50%-rate LDPC code rate is employed, the finial net rate or spectrum efficiency of a system according to the embodiment of the invention is the same as transmitting a QSPK signal (M=4). It is to be noted that, the high order modulation scheme is not limited to M-PSK or M-QAM modulation schemes, and other high order modulation schemes currently known or developed in the future may also be employed here.

The up-converter <b>26</b> up-converts the N_thigh-order-modulated data signals into the optical signal, which contains all the data to be transmitted, so that the optical signal is transmitted to the receiver <b>40</b>.

Next, the optical tone generator <b>22</b> will be described in detail. <figref idref="DRAWINGS">FIG. 2A</figref> is a block diagram showing the optical tone generator <b>22</b> illustrated in <figref idref="DRAWINGS">FIG. 1</figref>. As shown in <figref idref="DRAWINGS">FIG. 2A</figref>, the optical tone generator <b>22</b> comprises a first optical tone generating unit <b>31</b>, and a second optical tone generating unit <b>33</b> cascaded with the first optical tone generating unit <b>31</b>. An EDFA <b>32</b> is preferably arranged between the two optical tone generating units <b>31</b>, <b>33</b> to amplify the optical tones generated by the first optical tone generating unit <b>31</b> and then feed the amplified optical tones to the second optical tone generating unit <b>33</b>.

The first optical tone generating unit <b>31</b> generates multiple tones from the continuous-wave (CW) optical carrier generated by the optical source <b>21</b>. Spacing of the generated optical tones, i.e. a frequency reference of the first optical tone generating unit <b>31</b>, may be provided by a first clock source <b>35</b>, which may be a radio frequency (RF) frequency synthesizer. For example, some optical components or devices in the first optical tone generating unit <b>31</b> may be fine tuned to produce the multiple tones at different wavelengths from the single laser source. The first optical tone generating unit <b>31</b> may be an optical intensity modulator, and by performing certain adjustments on its bias voltage and input RF power of the optical intensity modulator, the first optical tone generating unit <b>31</b> may generate, for example, three optical tones with an even power level, as shown in <figref idref="DRAWINGS">FIG. 2B</figref>, in which case the tone spacing is 6.71875-GHz. The second optical tone generating unit <b>33</b> generates more (e.g. N_t) optical tones from the inputted optical tones. The second optical tone generating unit <b>33</b> may be a re-circulating frequency shifter (RFS), which includes an optical I/Q modulator, an EDFA, etc., as disclosed in Yiran Ma's post-deadline work in OFC'2009. Similar to the first optical tone generating unit, a frequency reference of the second optical tone generating unit <b>33</b> may be provided by a second clock source <b>37</b>. A common clock reference <b>36</b> is used to lock the two clock sources, so as to lock frequency stability for the clock sources. By applying the two cascaded optical tone generating units, a large number of optical tones may be effectively generated from a single laser source. In the 1.08-Tb/s example, the RFS tone generation unit <b>33</b> produces more than 17 optical tones individually if only one laser source is inputted, and the tone spacing is driven at 20.15625-GHz. When the three tones generated from the first optical tone generating unit are inputted, the RFS tone generation unit <b>33</b> can produce 51 (=3×17) tones. By tuning bandwidth and center wavelength of an optical filter in the optical tone generating unit, 50 optical tones with a signal-to-noise ratio more than 20 dB can be generated, which is shown in <figref idref="DRAWINGS">FIG. 2C</figref>. It is to be noted that, although an example of two optical tone generating units in the optical tone generator <b>22</b> is given above, the number of optical tone generating units is not limited to 2, and may be changed according to different requirements on optical tones and/or other factors. Specifically, only one optical tone generating unit may be used in the optical tone generator <b>22</b> to generate multiple optical tones. Alternatively, three or more optical tone generating units may be incorporated in the optical tone generator <b>22</b> to generate the optical tones. Additionally, the number of the generated optical tones may be one, and the types of the respective optical tone generating units are not limited to the intensity modulator or the RFS, and may be other types commonly known in the art. Accordingly, the configuration of the optical tone generator <b>22</b> may vary depending on different requirements of the optical tones and different generating methods.

Next, the up-converter <b>16</b> will be described with reference to <figref idref="DRAWINGS">FIG. 3</figref>. As shown in <figref idref="DRAWINGS">FIG. 3</figref>, the up-converter <b>26</b> includes N_tRF signal modulators <b>51</b> and associated N_toptical modulators <b>52</b>, and an optical coupler <b>53</b>.

The N_tRF signal modulators <b>51</b> receive the N_thigh-order-modulated data signals, and RF-modulate (or convert) the N_thigh-order-modulated data signals into N_tRF sub-band signals (in the electrical domain), respectively. Each of the RF sub-band signals occupies a part of band (a sub-band) of the whole band that the transmission of the data may occupy. In the embodiment, the RF sub-band signals may be single-carrier signals or orthogonal frequency division multiplexing (OFDM) signals. The manner of RF-modulating the N_thigh-order-modulated data signals is commonly known in the art, and thus will not be described in detail here.

The optical modulators <b>52</b> optically modulate the N_tRF sub-band signals onto corresponding optical tones provided from the optical tone generator <b>22</b>, respectively, so as to generate N_toptical sub-band signals (in the optical domain), and provide the optical sub-band signals into the optical coupler <b>53</b>.

The optical coupler <b>53</b> combines the N_toptical sub-band signals into an optical signal which occupies the whole band and contains all the data to be transmitted, and then outputs the optical signal, so as to transmit it via the optical transmission channel <b>30</b>.

The optical transmission channel <b>30</b> may be a fiber link formed by a standard single mode fiber (SSMF) with a length of for example 1000-km. One or more Erbium doped fiber amplifiers (EDFAs) (not shown in the figure) may be provided in the optical transmission channel <b>20</b> when necessary to amplify the transmitted optical signal so as to compensate the fiber loss during the transmission. In the 1.08-Tb/s example, the fiber link is emulated with a fiber loop with 80-km per span, which is a common experimental configuration; the 1040-km is emulated and controlled by a timer and two optical on/off switches; and one EDFA is placed in the loop to compensate the loss of each span.

The receiver <b>40</b> receives the optical signal transmitted from the transmitter <b>20</b>, and recovers the data from the optical signal. As shown in <figref idref="DRAWINGS">FIG. 1</figref>, the receiver <b>40</b> comprises a down-converter <b>41</b>, at least one (N_r) demappers <b>42</b>, and N_rdecoders <b>43</b>, where N_r≧1, and N_rmay be the same as or different from N_t.

The down-converter <b>41</b> down-converts the optical signal into N_rdata signals. Each of the data signals contains a part of data transmitted at the transmitting device. The down-converter <b>31</b> will be described in detail later.

Each of the N_rdemappers <b>42</b> performs high order demodulation, which corresponds to the high order modulation performed at the transmitter <b>20</b>, on respective one of the N_rdata signals. For example, when the transmitter <b>20</b> performs one of M-ray phase shift keying and M-ray quadrature amplitude modulation (M≧4), the demapper <b>42</b> performs corresponding demodulation according to the modulation order M. Additionally, the output of the demapper <b>42</b> determines the decision scheme used in the decoder <b>43</b>. Specifically, if the demapper <b>42</b> outputs binary data bits, then a hard-decision scheme is used in the decoder <b>43</b>. If likelihood for the binary data bits is outputted, then the decoder <b>43</b> will use a soft-decision scheme, which has further improvement compared to the hard-decision scheme. In the 1.08-Tb/s example, when the 16-QAM signal is demapped, the likelihoods of the decision are fed into the advanced decoder.

Subsequently, each of the N_rdecoders <b>43</b> performs advanced decoding, which corresponds to the advanced coding performed at the transmitter <b>20</b>, on respective one of the high-order-demodulated data signals outputted from the corresponding demapper <b>42</b>, so as to recover the data contained therein. Specifically, the decoder <b>43</b> decodes the data signal using a decoding scheme having the same rate and type as those of the coding scheme used in the transmitter. For instance, if the LDPC coding scheme is used in the transmitter, the decoder <b>43</b> performs the decoding by using several corresponding LDPC decoding algorithms, such as a log-domain sum-product algorithm, with the same coding rate. The outputs from the respective decoders <b>43</b> are the recovered data. BER is calculated by comparing such data with originally transmitted data in the transmitter <b>20</b>. In the 1.08-Tb/s example, the LDPC decoder is using log-domain sum-product algorithms.

Next, the down-converter <b>41</b> will be described with reference to <figref idref="DRAWINGS">FIG. 4</figref>.

As shown in <figref idref="DRAWINGS">FIG. 4</figref>, the down-converter <b>41</b> comprises a separator <b>54</b>, a down-converting unit <b>55</b>, and N_rRF signal demodulators <b>56</b>.

The separator <b>54</b> separates the optical signal which occupies the whole band into N_roptical sub-band signals, each of which occupies a part of the whole band, and provides the N_roptical sub-band signals to the down-converting unit <b>55</b>, respectively. For example, the optical sub-band separator <b>54</b> may use N_rband-pass filters to separate the optical signal, each band-pass filter selecting and outputting one optical sub-band signal.

The down-converting unit <b>55</b> down-converts the respective optical sub-band signals into N_rRF sub-band signals, which will be described later, and provides the N_rRF sub-band signals to corresponding RF signal demodulators <b>56</b>.

The RF signal demodulators <b>56</b> RF-demodulate (or convert) the N_rRF sub-band signals provided from the down-converting unit <b>55</b> into N_rdata signals, and provide the data signals to the downstream demapper <b>42</b>.

In the following, the down-converting unit <b>55</b> will be described with reference to <figref idref="DRAWINGS">FIG. 5</figref>. As shown in <figref idref="DRAWINGS">FIG. 5</figref>, the down-converting unit <b>55</b> includes a local frequency generator <b>57</b>, and at least one group of a polarization diversity optical hybrid <b>58</b> and optical photo-diodes <b>59</b>.

The optical sub-band signals outputted from the separator <b>55</b> are inputted to the respective polarization diversity optical hybrids <b>58</b>, as shown in <figref idref="DRAWINGS">FIG. 5</figref>. The local frequency generator <b>57</b> generates N_rlocal oscillating signals at N_rfrequencies, each of which is tuned close to a center of a sub-band of the optical sub-band signal inputted into the corresponding polarization diversity optical hybrid <b>58</b>, and provides the local oscillating signals to the respective polarization diversity optical hybrids <b>58</b>. The local frequency generator <b>58</b> may generate the local oscillating signals in various manners. For example, the local frequency generator may contain N_rlaser sources, each of which generates one local oscillating signal at one of the N_rfrequencies. The local frequency generator <b>57</b> may also use a structure similar to the multiple-tone generation structure described in <figref idref="DRAWINGS">FIG. 2</figref> followed by a tone separator for separating the tones. Alternatively, the local frequency generator <b>57</b> may use a combination of the multiple-tone generation structure and one or more laser sources to generate the local oscillating signals. If laser sources are used in the local frequency generator <b>57</b>, the number of the laser sources is not necessary to be one, and may be more than one. Additionally, although only one local frequency generator which generates N_rlocal oscillating signals is shown in <figref idref="DRAWINGS">FIG. 5</figref>, this is only illustrative, and instead, one or more local frequency generators may be used to generate the N_rlocal oscillating signals. In other words, any unit that can generate the multiple local oscillating signals can be used as one embodiment.

Each of the oscillating signals beats with the optical sub-band signal in the corresponding polarization diversity optical hybrid <b>58</b>, and then is detected by for example four pairs of balanced photo-diodes <b>59</b> in a coherent heterodyne fashion, so that each optical sub-band signal is converted into a RF sub-band signal. The particular manner for converting each optical sub-band signal into the RF sub-band signal by the local frequency generator, the polarization diversity optical hybrid, and the photo-diodes is well known in the art, thus detailed descriptions thereof are omitted here for simplicity. Additionally, the down-converting unit <b>56</b> may convert the optical sub-band signal in other manners, besides the one shown in <figref idref="DRAWINGS">FIG. 5</figref>.

In the above embodiment, by applying both the advanced coding and the high order modulation on the data signals, the sensitivity of the optical communication system is improved, thus a high speed long-haul optical transmission may be achieved. For example, it has been proved experimentally that a 1.08-Tb/s optical error-free transmission per single channel over a 1040-km SSMF fiber may be realized. <figref idref="DRAWINGS">FIG. 6</figref> shows a table presenting the BER performance for 1.08-Tb/s coherent optical OFDM over a 1040-km SSMF transmission under OSNR of 28.7 dB. It can be seen from this table that before decoding, the BER presents a very high error level, whereas after LDPC decoding, all the 50 tones can be fully recovered with no error.

Hereinafter, an optical communication method, a method for transmitting an optical signal, and a method for receiving an optical signal according to a first embodiment of the invention will be described.

In the optical communication method according to the first embodiment of the invention, the transmitter <b>20</b> transmits an optical signal containing data via an optical transmission channel, and then the receiver <b>40</b> receives the optical signal and recovers the data.

<figref idref="DRAWINGS">FIG. 7</figref> shows a flow chart of the method of transmitting the optical signal according to the first embodiment of the invention, which may be used in the transmitter <b>20</b>.

As shown in <figref idref="DRAWINGS">FIG. 7</figref>, in step <b>60</b>, N_toptical tones are generated. The N_roptical tones may be generated by at least one optical tone generating unit from a single laser source, as shown in <figref idref="DRAWINGS">FIG. 2</figref>.

In step <b>61</b>, advanced coding is performed on N_tdata signals respectively, each of the N_tdata signals carrying a part of the data to be transmitted. The N_tdata signals are the same as those described with reference to <figref idref="DRAWINGS">FIG. 1</figref>. The advanced coding is preferably coding with a net coding gain above 5 dB at BER=10⁻¹³compared to BER-versus-OSNR performance of un-coded transmission, and may have a net coding rate between 20%-85%. Examples of the advanced coding are low density parity-check coding or Turbo coding. It is to be noted that, the advanced coding scheme is not limited to the low-density parity-check coding scheme and the Turbo coding scheme, and other advanced error correction coding scheme currently known or developed in the future may also be employed as the advanced coding scheme.

In step <b>62</b>, high order modulation is performed on the N_tcoded data signals. The high order modulation performed here may be one of M-ray phase shift keying and M-ray quadrature amplitude modulation, M≧4. Other high order modulation scheme currently known or developed in the future are also applicable.

In step <b>63</b>, the N_thigh-order-modulated data signals are up-converted into the optical signal to be outputted through the at least optical tone generated in step <b>60</b>. Specifically, the N_thigh-order-modulated data signals are RF-modulated into N_tRF sub-band signals, which may be single-carrier signals or OFDM signals as described above; then the N_tRF sub-band signals are optically modulated onto the respective optical tones generated in step <b>60</b> to generate N_toptical sub-band signals; finally, the N_toptical sub-band signals are combined into the optical signal, so that the optical signal is outputted.

The method of receiving the optical signal according to the first embodiment of the invention, which may be used in the receiver <b>40</b>, will be described with reference to <figref idref="DRAWINGS">FIG. 8</figref>.

As shown in <figref idref="DRAWINGS">FIG. 8</figref>, in step <b>65</b>, the optical signal received from a transmitter is down-converted into N_rdata signals. Specifically, the optical signal is separated into N_roptical sub-band signals, then the N_roptical sub-band signals are down-converted into N_rRF sub-band signals, respectively; finally, the N_rRF sub-band signals are RF-demodulated into N_rdata signals. As described above, the RF signal demodulation may use either single-carrier or OFDM coherent detection schemes, depending on the RF signal modulation scheme used in the transmitter.

High order demodulation, which corresponds to the high order modulation performed at step <b>62</b>, is performed on the N_rdata signals in step <b>66</b>. For example, when one of M-ray phase shift keying and M-ray quadrature amplitude modulation (M≧4) is performed at the transmitter, the demodulation is performed correspondingly here according to the modulation order M.

In step <b>67</b>, advanced decoding, which corresponds to the advanced coding performed in step <b>61</b>, is performed on the N_rhigh-order-demodulated data signals so as to recover the data originally transmitted. The decoding scheme used here may have the same rate and type as those of the coding scheme used in the transmitter, as described above.

In the first embodiment of the invention, the data signals, each of which contains a part of data to be transmitted, are encoded and high-order-modulated before optically modulated onto the optical tones. In such solution, an error floor may be caused by the encoder, and an effect of burst errors occurring in the transmission and other adverse effects may also exist. To eliminate these adverse effects, a second embodiment of the invention is proposed.

<figref idref="DRAWINGS">FIG. 9</figref> shows a schematic configuration of a high speed (e.g. over 1-Tb/s) long-haul (e.g. over 1000-km) optical communication system according to a second embodiment of the present invention.

As shown in <figref idref="DRAWINGS">FIG. 9</figref>, the optical communication system <b>70</b> comprises a transmitter (also referred to as a transmission device or a transmitting device) <b>80</b>, the optical transmission channel <b>30</b>, and a receiver (also referred to as a receiving device) <b>90</b>. The transmitter <b>80</b> according to the second embodiment of the invention is described.

The transmitter <b>80</b> transmits an optical signal containing data to the receiver <b>90</b>, and comprises an optical source <b>81</b>, an optical tone generator <b>82</b>, and an optical signal generator <b>83</b>.

The optical source <b>81</b>, which may be a single laser source, generates a CW optical carrier at a predetermined wavelength, and the optical tone generator <b>82</b> generates N_toptical tones from the single optical source. The optical signal generator <b>83</b> generates the optical signal, and then transmits the optical signal to the receiver <b>90</b> via the optical transmission channel <b>30</b>. The optical source <b>81</b> and the optical tone generator <b>82</b> in the transmitter <b>80</b> are respectively the same as the optical source <b>21</b> and the optical tone generator <b>22</b> described above, thus descriptions thereof are omitted here for simplicity.

<figref idref="DRAWINGS">FIG. 10</figref> is a block diagram showing the optical signal generator <b>83</b>. As shown in <figref idref="DRAWINGS">FIG. 10</figref>, the optical signal generator <b>83</b> includes an optical tone separator <b>101</b>, N_tRF sub-band signal generating units <b>102</b> and N_tassociated optical modulator <b>103</b>, and an optical coupler <b>104</b>.

The optical tone separator <b>101</b> separates the N_toptical tones provided from the optical tone generator <b>82</b> into N_tchannels (N_tsub-bands), and provides each of the optical tones into a corresponding one of the optical modulators <b>103</b>. For example, the optical tone separator <b>101</b> may include a plurality of band-pass filters, each of which filters the N_toptical tones so as to select one of them in each channel, and provides it to a corresponding optical modulator <b>103</b>.

The N_tRF sub-band signal generating units <b>102</b> generate N_tRF sub-band signals carrying all data to be transmitted, each of which carries a part of the data. Specifically, each of the RF sub-band signal generating units <b>102</b> generates one RF sub-band signal, which may be a single-carrier data signal or an OFDM data signal, and outputs the RF sub-band signal into an associated optical modulator <b>103</b>. Then, the optical modulator <b>103</b> optically modulates the RF sub-band signal onto a corresponding optical tone provided from the optical tone separator <b>101</b> to form an optical sub-band signal, i.e. up-converts the RF sub-band signal from the electrical domain to the optical domain. Thus, each optical sub-band signal occupies a part of the whole signal band.

The optical coupler <b>104</b> combines the N_toptical sub-band signals outputted from the respective optical modulators into the optical signal and outputs the optical signal.

In the following, the sub-band signal generating units <b>102</b> will be described in detail below with reference to <figref idref="DRAWINGS">FIG. 11</figref>. As shown in <figref idref="DRAWINGS">FIG. 11</figref>, each of the sub-band signal generating unit <b>102</b> comprises an outer encoder <b>111</b>, an interleaver <b>112</b>, an inner encoder <b>113</b>, a mapper <b>114</b>, a RF signal modulator <b>115</b>, and digital-to-analog converters (DACs) <b>116</b>.

As described above, the sub-band signal generating units <b>102</b> generate RF sub-band signals each of which contains a part of data to be transmitted. Therefore, a data source (not shown in this figure) provides each of the data signals to a respective one of the sub-band signal generating units <b>102</b> and thus to the outer encoder <b>111</b> therein.

The outer encoder <b>111</b> encodes the data signal so as to eliminate a potential error floor which may be caused by the downstream inner encoder <b>113</b>. As an example, the outer encoder <b>111</b> may use Reed-Solomon RS(255, 239) to encode the data signal, so as to correct randomly distributed errors with a bit error ratio (BER) under ˜2×10⁻³.

The interleaver <b>112</b> is arranged between the outer encoder <b>111</b> and the inner encoder <b>113</b>, and interleaves the outer-coded data signal outputted from the outer encoder <b>111</b> so as to avoid an effect of burst error.

The inner encoder <b>113</b> performs advanced coding on the interleaved data signal, and then provides the coded data signal to the mapper <b>114</b>. The advanced coding performed by the inner encoder <b>113</b> is the same as that in the first embodiment, and thus will not be described here for simplicity.

The mapper <b>114</b> maps the coded data signal onto high order modulation, i.e. performs high order modulation on the coded data signal. The high order modulation performed by the mapper <b>114</b> is the same as that in the first embodiment, and thus will not be described here for simplicity.

The RF signal modulator <b>115</b> is actually a single-carrier or OFDM signal generating unit, and generates a RF sub-band signal (single-carrier or OFDM signal) from the high-order-modulated data signal, as described with reference to <figref idref="DRAWINGS">FIG. 3</figref>. The generated RF sub-band signal is outputted to the digital-to-analog converters (DACs) <b>116</b> to be converted into an analog sub-band signal and then outputted.

Hereinafter, the receiver <b>90</b> according to the second embodiment of the embodiment will be described with reference to <figref idref="DRAWINGS">FIGS. 9 and 12</figref>.

The receiver <b>90</b> receives the optical signal and recovers the data therefrom. As shown in <figref idref="DRAWINGS">FIG. 9</figref>, the receiver <b>90</b> comprises a separator <b>91</b> which separates the optical signal to N_roptical sub-band signals, a down-converting unit <b>92</b> which down-converts the N_roptical sub-band signals into N_rRF sub-band signals (in the electrical domain), respectively, and N_rsignal processors <b>93</b> for down-converting the N_rRF sub-band signals into N_rdata signals, and performing advanced decoding, which corresponds to the advanced coding performed at the transmitting device, and high order demodulation, which corresponds to the high order modulation performed at the transmitter, on the N_rdata signals respectively so as to recover the data originally transmitted.

The separator <b>91</b> and the down-converting unit <b>92</b> are the same as the separator <b>54</b> and the down-converting unit <b>55</b>, thus descriptions thereof are omitted here for simplicity.

As shown in <figref idref="DRAWINGS">FIG. 12</figref>, each of the signal processors <b>93</b> comprises one or more analog-to-digital converters (ADCs) <b>121</b> (only one ADC is shown in <figref idref="DRAWINGS">FIG. 12</figref>), a RF signal demodulator <b>122</b>, a demapper <b>123</b>, an inner decoder <b>124</b>, a de-interleaver <b>125</b>, and an outer decoder <b>126</b>.

The ADCs <b>121</b> may be one or more than one high speed ADCs, and are used to sample the high speed analog sub-band signal outputted from the corresponding polarization diversity optical hybrid in the down-converting unit <b>62</b>, so as to convert the analog sub-band signal into a digital RF sub-band signal.

As described above, the RF sub-band signal generated at the transmitter <b>80</b> may be a single-carrier signal or an OFDM signal. Correspondingly, the RF signal demodulator <b>122</b> RF-demodulates the RF sub-band signal into a data signal by using either single-carrier or OFDM coherent detection schemes. Preferably, modules for performing several DSP procedures, such as frequency offset estimation, channel estimation, phase noise estimation, etc., are further integrated in the RF signal demodulator <b>122</b> to eliminate noises in the signal and thus improve quality of the signal.

The demapper <b>123</b> performs high order demodulation, which corresponds to the high order modulation performed at the transmitter, on the data signal outputted from the RF signal demodulator <b>122</b>, so as to detect the signal from the constellations constructed in the RF signal demodulator <b>122</b>. For example, when the transmitter <b>80</b> performs one of M-ray phase shift keying and M-ray quadrature amplitude modulation (M≧4), the demapper <b>123</b> performs corresponding demodulation according to the modulation order M.

The inner decoder <b>124</b> performs advanced decoding, which corresponds to the advanced coding performed at the transmitter, on the high-order-demodulated data signal, so as to recover the data bits contained therein. The high order demodulation performed by the inner decoder <b>124</b> is the same as that described in the first embodiments, and thus is not described in detail here.

The de-interleaver <b>125</b> de-interleaves the decoded signal in a manner corresponding to the interleaving manner employed in the interleaver <b>112</b>, so as to reconstruct the data contained therein.

The outer decoder <b>126</b> then decodes the reconstructed data in a decoding manner corresponding to the coding scheme employed in the outer encoder <b>111</b>.

The data outputted from the respective signal processors <b>93</b> are the recovered data corresponding to the data transmitted from the transmitter.

In the second embodiment of the invention, devices such as the outer encoder, the interleaver, the outer decoder, the de-interleaver, or the like are added into the system. Thus, the error floor which may be caused by the downstream inner encoder, and other adverse effects may be eliminated, the quality of the transmitted signal is further improved, and the sensitivity of the system is increased.

Hereinafter, an optical communication method, a method for transmitting an optical signal, and a method for receiving an optical signal according to a second embodiment of the invention is described. It is to be noted that, the steps in the second embodiments which are the same as those in the first embodiment will be described briefly or not described here for simplicity.

In the optical communication method according to the second embodiment of the invention, the transmitter <b>80</b> transmits an optical signal containing data via an optical transmission channel, and then, the receiver <b>90</b> receives the optical signal, and recovers the data.

<figref idref="DRAWINGS">FIG. 13</figref> shows a flow chart of the method of transmitting the optical signal according to the second embodiment of the invention, which may be executed in the transmitter <b>80</b>. As shown in <figref idref="DRAWINGS">FIG. 13</figref>, in step <b>130</b>, N_toptical tones are generated. Then, in step <b>131</b>, an outer coding, such as Reed-Solomon coding described above, is performed on N_tdata signals respectively, each of the N_tdata signals carrying a part of the data to be transmitted.

The N_tdata signals are interleaved in step <b>132</b>, and then in step <b>133</b>, advanced coding as described above is performed on the N_tdata signals. In step <b>134</b>, high order modulation is performed on the coded N_tdata signals. The high order modulation is the same as that described in the first embodiment, and thus will not be described in detail.

Next, N_tRF sub-band signals are generated from the respective high-order-modulated data signals in step <b>135</b>. As described above, the generated sub-band signals may be single-carrier signals or OFDM signals.

Then, after being subjected to a digital-to-analog conversion, in step <b>136</b>, the N_tRF sub-band signals are optically modulated onto the N_toptical tones generated in step <b>130</b>, so as to generate N_toptical sub-band signals. Finally, the N_toptical sub-band signals are combined into the optical signal to be outputted in step <b>137</b>.

Now, the method of receiving an optical signal according to the second embodiment of the invention, which may be executed in the receiver <b>80</b>, is described with reference to <figref idref="DRAWINGS">FIG. 14</figref>.

Steps <b>140</b> to <b>144</b> shown in <figref idref="DRAWINGS">FIG. 14</figref> are respectively similar to steps <b>65</b> to <b>67</b> shown in <figref idref="DRAWINGS">FIG. 8</figref>, and thus are described in brief here.

In step <b>140</b>, the received optical signal is separated into N_roptical sub-band signals, each of which occupies a sub-band of the whole band that the transmission of the signal occupies. In step <b>141</b>, the N_roptical sub-band signals are down-converted into N_rRF sub-band signals. In step <b>142</b>, the N_rRF sub-band signals are RF-demodulated into N_rdata signals, respectively. In step <b>143</b>, high order demodulation, which corresponds to the high order modulation performed at step <b>134</b>, is performed on the N_rdata signals. Subsequently, in step <b>144</b>, advanced decoding, which corresponds to the advanced coding performed in step <b>133</b>, is performed on the N_rhigh-order-demodulated data signals.

In step <b>145</b>, the decoded data signals are de-interleaved in a manner corresponding to the interleaving manner used in step <b>132</b>. Then, in step <b>146</b>, outer decoding, which corresponds to the outer coding performed in step <b>131</b>, is performed on the de-interleaved data signals, so as to recover the data contained therein. Thus, the data originally transmitted may be obtained.

By the transmitting device, the receiving device, the optical communication system, and the associated methods according to the present invention, high speed (e.g., over 1 Tb/s) transmission per single channel over a long-haul distance (e.g. over 1000-km) with error-free recovery may be achieved.

The embodiments of the invention may be implemented in hardware, software, firmware, or a combination thereof. For example, in the signal processor <b>93</b> of the second embodiment of the invention, the ADC <b>121</b> may be implemented in hardware, and the other components may be implemented in software.

While exemplary embodiments of the present invention are described here, these embodiments are shown by way of example. It should be understood that there is no intent to limit exemplary embodiments of the present invention to the particular forms disclosed. On the contrary, exemplary embodiments are to cover all modifications, equivalents, and alternatives falling within the scope of the present invention.

It will be understood that, although the terms first, second, etc. may be used herein to describe various elements, these elements should not be limited by these terms. These terms are only used to distinguish one element from another. For example, a first element could be termed a second element, and, similarly, a second element could be termed a first element, without departing from the scope of exemplary embodiments of the present invention. As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items.

It will be understood that when an element is referred to as being “connected” or “coupled” to another element, it can be directly connected or coupled to the other element or intervening elements may be present. In contrast, when an element is referred to as being “directly connected” or “directly coupled” to another element, there are no intervening elements present. Other words used to describe the relationship between elements should be interpreted in a like fashion (e.g., “between” versus “directly between” “adjacent” versus “directly adjacent” etc.).

The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of exemplary embodiments of the present invention. As used herein, the singular forms “a,” “an,” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. As used herein, the terms “and/or” and “at least one of” include any and all combinations of one or more of the associated listed items. It will be further understood that the terms “comprises,” “comprising,” “includes,” and/or “including” when used herein, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof.

It should also be noted that in some alternative implementations, the functions/acts noted may occur out of the order noted in the figures. For example, two figures shown in succession may in fact be executed substantially concurrently or may sometimes be executed in the reverse order, depending upon the functionality/acts involved.

While some exemplary embodiments of the present invention have been described, it will be obvious that the same may be varied in many ways. The description of the present invention hereinbefore uses these examples, including the best mode, to enable any person skilled in the art to practice the present invention, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the present invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal languages of the claims. Such variations are not to be regarded as a departure from the spirit and scope of the present invention, and all such modifications are intended to be included within the scope of the present invention as stated in the following claims. What is claimed is: